ci(coverage): Pin toolchain to nightly-2025-11-27

This fixes a CI failure resulting from use of rustc nightly
with `llvm-cov`.

refer to:
https://github.com/bitcoindevkit/bdk/actions/runs/19946035484.

As a workaround we change `code_coverage.yml` to set the toolchain
to `nightly-2025-11-27` which is a version of the compiler that
is known to work with `cargo-llvm-cov` at generating coverage
reports.

Author: ValuedMammal

.github/workflows/code_coverage.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Install Rust toolchain
         uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
-            toolchain: nightly
+            toolchain: nightly-2025-11-27
             override: true
             cache: true
             components: llvm-tools-preview
@@ -27,7 +27,7 @@ jobs:
       - name: Make coverage directory
         run: mkdir coverage
       - name: Test and report coverage
-        run: cargo +nightly llvm-cov -q --doctests --branch --all --ignore-filename-regex "(example*|crates/testenv/*)" --all-features --lcov --output-path ./coverage/lcov.info
+        run: cargo llvm-cov -q --doctests --branch --all --ignore-filename-regex "(example*|crates/testenv/*)" --all-features --lcov --output-path ./coverage/lcov.info
       - name: Generate HTML coverage report
         run: genhtml -o coverage-report.html --ignore-errors unmapped ./coverage/lcov.info
       - name: Coveralls upload